PROJECT SUMMARY Pulmonary fibrosis represents an increasing cause of mortality worldwide and despite decades of investigation, considerable uncertainty exists as how this disease initiates and progresses. While multiple fibrogenic molecules have been found to drive aberrant matrix deposition, the mechanisms responsible for maintaining persistent and self-sustaining fibrogenesis are largely unknown. Targeting mechanisms that perpetuate the pathological state of fibroblasts during disease progression may serve as an attractive therapeutic strategy to halt lung fibrosis. The current proposal addresses the role of epigenetic gene repression in regulating fibroblast activation and lung fibrosis development. We will investigate the role of histone 3 lysine 9 methylation (H3K9me) as an important epigenetic modification that represses the transcription of genes essential to maintaining or returning lung fibroblasts to an anti-fibrotic or quiescent inactive state. Our preliminary data demonstrate that inhibition of H3K9 methylation by targeting the H3K9 methyltransferase G9a or the epigenetic reader CBX5 potently inhibits fibroblast activation by fibrogenic stimuli. Mechanistically, our data demonstrate that both G9a and CBX5 are directly involved in repressing PGC1?, a master regulator of mitochondria metabolism significantly downregulated in diseased lung fibroblasts. Loss of PGC1? expression promotes fibroblast activation, while restoring PGC1? via epigenetic mechanisms reverses fibroblast activation. We will use loss of function strategies to target the epigenetic regulators CBX5 and G9a to investigate their mechanistic roles in switching fibroblasts between activated and quiescence states. Using mouse genetics approaches we will investigate the benefits of inhibiting H3K9 methylation in halting disease progression in bleomycin-induced lung fibrosis models. As our preliminary data strongly support an anti-fibrotic function for PGC1? during lung fibroblast activation in vitro, in this proposal we will further characterize its anti-fibrotic function and evaluate upstream and downstream transcriptional network that mediate its anti-fibrotic functions. Taken together, the proposed research studies will reveal critical epigenetic targets for therapeutic interventions aimed at halting or reversing the progression of pulmonary fibrosis.